The corresponding prior art pump is described below in order to specify the function of the non-return valve of the present invention. This description also serves to show up the shortcomings of admission valves as used in the past. The description is made with reference to a longitudinal section in FIG. 1 of the accompanying drawings. This figure shows a precompression pump intended for crimping in sealed manner onto a can (not shown) by means of a capsule 10 and gaskets 9a and 9b. When actuated, the pump must be held in the vertical position, as shown.
Thus, the prior art precompression pump is essentially protected by a pump cylinder 1. The narrow end 12 of the cylinder 1 communicates with the supply of liquid to be emitted (generally via a dip tube, not shown). Between the end 12 and the hollow body 11 of the cylinder 1 there is a non-return valve constituted in the example shown by a disk-shaped gasket 3 and a sleeve 2 for guiding the gasket 3. A piston 5 is free to move inside the body 11. The piston is provided with outer sealing lips 5a. Together with the non-return valve, these lips define a pump chamber 19 which is thus isolated in the bottom portion of the body 11. The piston 5 has a central opening 5b which receives a rod 4 having a blind channel 4a therein which opens out sideways via an orifice 4b. The rod 4 and the piston 5 are designed so that a spring 7, when compressed, allows the rod 4 to move relative to the piston 5. Thus, the orifice 4b may move from the position shown in FIG. 1 where it is closed by the piston 5 to a position in which it is communication with the pump chamber 19. Finally, within the pump chamber there are a return spring 8 which is less stiff than the spring 7, and a ring 6 whose essential function is to ensure very good sealing between the piston 5 and the rod 4.
When a user presses down the rod 4, this gives rise initially to the piston 5 being lowered within the body 11 of the pump cylinder 1 under drive from the spring 7. However, this movement remains extremely limited. The gasket 3 bears against an upstanding lip la projecting from the base of the body 11 and isolating the pump chamber 19. The incompressibility of the liquid contained therein therefore stops the stroke of the piston 5 while the pressure inside the chamber 19 increases. The main effect of the user's action is thus to compress the spring 7. This occurs when the pressure in the pump chamber 19 reaches a certain threshold relative to the stiffness of the spring 7. This threshold is the reason for the pump being referred to as a "precompression" pump since, once the threshold is exceeded, the chamber 19 is put into communication with the outside via the orifice 4b and the channel 4a, thereby allowing the liquid to flow. It is then the turn of the spring 8 to be compressed while the volume of the chamber 19 is reduced. Liquid continues to be emitted until the base 6a of the ring 6 comes into abutment against the surface of the sleeve 2.
The user then ceases to press down the rod 4. The stiffer spring 7 is the first to return to its initial position, thereby raising the rod 4 through the opening 5b in the piston 5. As a result the path to the outside via the hole 4b and the channel 4a closes. Thereafter, the spring 8 expands in turn and the pressure inside the pump chamber 19 falls. This causes the gasket 3 to be sucked up to the top of the guide sleeve 2, thereby putting the chamber 19 into communication with the inside of the liquid-containing can. The liquid is thus admitted into the chamber 19 by passing round the gasket 3. Ribs 2b on the top of the sleeve 2 serve to prevent the gasket 3 from closing the central hole 2a through the sleeve. Once the pump chamber 19 has returned to its original size, it is again filled with liquid and the pressure on either side of the gasket 3 comes back into equilibrium. With the prior art non-return valve shown in FIG. 1, it is then hoped that gravity will return the gasket 3 to rest against the lip at the base of the body 11, thereby isolating the pump chamber 19 in order to prepare it for subsequent actuation.
However, the gasket 3 does not always return to its proper place, as expected. It often ends up at an angle inside the sleeve 2. Then, when the precompression pump is actuated again, the pump chamber 19 remains in communication with the supply of liquid to be sprayed. The pressure inside the chamber 19 therefore does not increase and as a result the spring 7 is not compressed so the liquid-emitting passage does not open. This faulty operation occurs above all when a gas is to be pumped (for example when priming the pump). In this case, the gasket 3a tends to remain stuck to the top of the sleeve 2 by a skin effect (e.g. due to the appearance of static electricity).
Naturally, from the user's point of view, the resulting random operation is hardly encouraging. Indeed, for pharmaceutical substances which need to be dispensed in accurate doses, this defect may proscribe using such a pump. The defect also produces a problem for the manufacturer of the pump. Pumps are tested prior to being sold, and the test commonly used for evaluating pump performance consists in actuating a pump three times when it is mounted on a supply of air. The pressure drop created in this way in the air supply is used as a criterion for distinguishing between good operation and bad. Thus, pumps in which the gasket 3 does not return properly in the presence of air during the test strokes are rejected even though such pumps would work perfectly well in the presence of the liquids for which they are designed. The high reject rate thus constitutes a significant loss of income for the manufacturer.
As a result, the admission valve of the present invention seeks to reduce the random nature of pump operation very considerably, particularly when the pump is operated in the presence of a gas. The gasket 3 must return every time into its proper place for effectively preventing communication between the pump chamber 19 and the inside of the can.